Well, when the 15th falls on a Sunday it's standard practice to move the deadline to Monday. But this year, Monday is also the District of Columbia's Emancipation Day, as NPR's Wendy Kaufman reminds us.

So, the deadline — for everyone, not just D.C. residents — gets bumped back another day.

"The enslaved people in Washington, D.C., were freed nine months before President Abraham Lincoln issued his famous Emancipation Proclamation, telegraphing the eventual end of slavery to the rest of the nation. On April 16, 1862, President Lincoln signed the Compensated Emancipation Act, for the release of certain persons held to service or labor in the District of Columbia, making D.C. residents the 'First Freed' by the federal government.

"Emancipation Day celebrations were held annually from 1866 through 1901, and resumed in 2002. In 2005 Emancipation Day was made an official public holiday in the District of Columbia. Each year, a series of educational and commemorative activities is held during the week prior to the holiday that celebrates the end of slavery in Washington, DC."